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Abstract: Mitochondrial genes are passed on in the female line only. Furthermore, in social hymenoptera, such as bees, wasps, and ants, daughters have (genetically) twice the reproductive value of males, because of the complementary ("haplo-diploid") sex determination system. Mitochondrial genes are also typically involved in coding for the pathways of the energy metabolism; this takes on special significance as defence against parasitic infections, for example, has been shown to be energetically costly. Despite these significant elements, the functional study of mitochondrial variation is almost absent in social insects. As a case study, we therefore investigated the distribution and dynamics of mitochondrial haplotypes (mitotypes) in natural populations of Bombus terrestris in a field population in Switzerland. Our data show a diversity of extant mitotypes with two types being dominant. A field experiment demonstrated that contrary to simple expectations of parasite-driven negative frequency-dependent selection, colony fitness surrogates were correlated with mitotype frequency. These findings are valued by reference to the literature, which shows that this subject matter is virtually unexplored as yet; especially, there is very little knowledge on the functional significance of different mitochondrial lines in one of the best studied groups of social insects – the ants, but see (Hasegawa & al. 2011) in this volume.